1. Field of the Invention
This invention relates generally to a control system for providing an AC traction motor with protection against damaging thermal overloads and, more particularly, to a system using information measured and supplied by a vehicle propulsion system controller to predict internal motor temperatures in near real time and to control the operation of the motor's inverter and blower fan.
2. Description of the Related Art
Alternating current (AC) traction motors are conventionally used in conjunction with electronic inverter drives as a means for transportation vehicle propulsion. The traction motor's capability to operate outside its continuous operating envelope for a short period of time is typically used to obtain optimum vehicle performance; however, the overheating which results reduces the motor life expectancy. Under these conditions, thermal protection is necessary to prevent damage of the stator winding insulation system and rotor cage.
Overload protection for rotating electrical machinery is intended to provide a means of preventing excessive overheating of the electrical winding system. Embedded winding sensors are used in conventional thermal protection schemes. Most large machines used in industrial applications generally employ resistance temperature detectors (RTDs) embedded in the stator windings for the purpose of directly sensing stator winding temperatures.
A more sophisticated technique is described in David R. Boothman et al., "Thermal Tracking--a Rational Approach to Motor Protection," IEEE Transactions on Power Apparatus and Systems, Vol. PAS-93 (Sept.-Oct. 1974), pp. 1335-1344. The Boothman article describes an overload analog relay with a design based on knowledge of the rotating machine's thermal circuit and feedback from an embedded winding detector. The system tracks motor temperature and provides a disconnect signal if a limit condition is exceeded.
Historically AC traction motors have not been protected against damaging thermal overloads. The location of the motors exposes temperature sensor lead cables to impacts from track debris and hostile environmental conditions. The possibility of false indications or damage to the sensor is usually considered a system reliability issue which precludes the use of embedded winding sensors.